Usually I write about mistakes that other writers have made. In order to be a bit more fair, I think it's time to write about mistakes that I myself have made recently.
So, when it comes to making mistakes, one that really annoys me is when your script as a whole doesn't work because certain scenes are in the wrong order.
This happened to me with my Modern Family spec 'House of Cards'. When I sent it to a competition, I rushed the script, didn't rewrite it early enough, made some panicky changes and screwed up the pacing without even realizing it.
Here's the script that nevertheless made the top 10 in the competition : http://www.scribd.com/read/116264736/Modern-Family-House-of-Cards
Now, the problem with this script is that the storyline with Alex and Haley ends too early. This is because I didn't pay enough attention to the scenes (placing) involving Jay/Phil and Cam/Mitchell. I thought the script would work no matter what.
Of course in hindsight it really didn't work and this is a good reminder that there's really no upside to start rewriting your script five hours before the deadline when you haven't checked your draft in months.
The upside to this all is that I think it's pretty easy to fix the problem - as long as the pacing is what's wrong with the script. The downside is that you start doubting yourself even more. Mistakes are never easy to accept anyway.
So, nevertheless, I opened the script yesterday and did some changes to it. The Claire/Haley/Alex storyline now ends on page 20 instead of ending on page 17. I don't know if it makes enough difference but I hope it does.
Anyway, here's the 'fixed' version of the spec script: http://www.scribd.com/doc/179356159/Modern-Family-House-of-Cards-Fix-pdf